Micropumps, i.e. pumps adapted for providing flow rates for liquids in the range of 1 μl/hour to 1 ml/min as such are well known in the art (although it should be noted that the indicated range is not per se a definition of a micropump). For example, an early micropump was proposed by H. van Lintel et al. in “A piezoelectric micropump based on micromachining of silicon” (Sensors and Actuators, 15, 1988, pp. 153-157), the pump comprising a machined silicon plate placed between two glass plates and shifted by a piezoelectric element.
More specifically, the silicon plate is etched to form a cavity, which, with one of the glass plates, defines the pumping chamber, an inflow or suction valve and at least one outflow or expelling valve, allowing the pumping chamber to communicate respectively with an inflow channel and an outflow channel. The part of the plate forming a wall of the pumping chamber can be deformed by a control member provided for example as a piezoelectric element. The same is equipped with two electrodes which, when they are connected to a source of voltage, cause the deformation of the element and, consequently, the deformation of the plate, which causes a variation of the volume of the pumping chamber. This movable or deformable wall of the pumping chamber can thus be moved between two positions.
The functioning of the micropump is as follows. When no voltage is applied to the piezoelectric chip, the inlet and outlet valves are in their closed position. When a voltage is applied, an increase of the pressure inside the pumping chamber occurs, which causes the opening of the outlet valve. The fluid contained in the pumping chamber is then expelled through the outflow channel by the displacement of the deformable wall from a first position towards a second position. During this phase, the inlet valve is maintained closed by the pressure prevailing in the pumping chamber. Conversely, when the voltage is decreased, the pressure in the pumping chamber decreases. This causes the closing of the outlet valve and the opening of the inlet valve. The fluid is then sucked into the pumping chamber through the inflow channel, owing to the displacement of the deformable wall from the second position to the first position. As normally passive valves are used, the actual design of the valve will determine the sensitivity to external conditions (e.g. back pressure) as well as the opening and closing characteristics thereof, typically resulting in a compromise between the desire to have a low opening pressure and a minimum of backflow. As also appears, a membrane micropump functions as any conventional type of membrane pump, for example described for use as a fuel pump in U.S. Pat. No. 2,980,032.
One disadvantage with this type of micropump is that the silicon membrane's warping is slight in comparison with the size of the pump chamber, this making the pump less suitable for pumping of gas. Although the need for pumping gas as such may not be relevant in many fields of use, in many of the above-mentioned applications, it would be advantageous that the pumps be self-priming. To be able to draw in liquids in a pump initially filled only with air, a sufficiently high negative pressure must be generated when operating with air. Additionally, it may be required that the pumps also be self-filling, i.e. that no gas bubbles remain in the pump which would impair pump performance. Further, the manufacturing costs for silicon-based micropumps are very high, making this technology at present unsuitable for a disposable pump.
Addressing these problems, U.S. Pat. No. 5,725,363 (B. Büstgens et al.) discloses a micromembrane pump which comprises a lower housing, an upper housing and a pump membrane situated between them, with the membrane providing the inlet and outlet valve functions as well, operating together with the valve seat integrated with the housing. The pump membrane manufactured from polyimide is shifted by thermal expansion of a gaseous medium or by phase transition of a liquid medium to its gaseous state in the actuator chamber.
In the disclosed embodiment, a heating element is formed integrally with the pump membrane using a thin-layer-technology.
As indicated above, micropumps may be used in particular for the administration of medicinal drugs. It is therefore important that the flow rate of the micropump be well defined, so that the medical drug to be infused is metered very precisely. However, the above-described micropumps suffer in this respect, from certain imperfections.
More specifically, the flow rate of a micropump depends on the variation of the volume of the pumping chamber between the two end positions of the moving membrane. This variation of the volume depends on several parameters. For example, for a piezoelectric driven membrane the voltage applied to the piezoelectric element, the physical characteristics of the piezoelectric element (thickness, diameter, dielectric constant) and of the pump membrane (material, thickness) may influence the volume. Thus, the same voltage applied to micropumps apparently identical may cause differing deformations of the pumping chamber of these micropumps, which, subsequently, will produce differing flow rates. Correspondingly, for a heat driven pump, heat transfer through the pump membrane to the fluid to be pumped as well as to the surroundings will influence the accuracy of the pump. Furthermore, for a given micropump, the flow rate can drift in the course of time due to aging of the materials from which the piezoelectric chip is made and the aging of the adhesive used for its bonding. Finally, the flow rate of the micropump depends on the pressure in the outflow and inflow channels. Indeed, it would be possible to incorporate additional metering means, e.g. based on thermodilution as disclosed in EP 1 177 802 (Becton, Dickinson and Company).
Addressing these problems, U.S. Pat. No. 5,759,015 (H. van Lintel et al.) discloses a silicon based micropump incorporating first and second stopper members arranged in such a manner as to limit the amplitude of the movement of the pump membrane in its opposite directions, with the first stopper members limiting this movement during the sucking of the fluid inside the pumping chamber and the second stopper members limiting this movement during the expelling of fluid from the pumping chamber. Although the stopper members help to improve the accuracy of the pump, the movements of the pump membrane inherently relies on the manufacturing accuracy of both the stopper members and the wall portions which they abut. Based on this pump design, pumps have been developed which are described as self-priming (see for example D. Maillefer et al, “A high-performance silicon micropump for disposable drug delivery systems”, Debiotec SA, Switzerland), however, as discussed above, a silicon based design still suffers from the disadvantage of high manufacturing costs.
A further problem with the silicon and polyimide based membrane pumps is the small stroke used in these pumps. For a metering pump this requires very fine tolerances which may be achieved using etching technologies, however, for moulded components it is difficult and/or expensive to ensure such fine tolerances.
Although the above-described micropump is capable of pumping gas as well as liquids and thus in principle is both self-filling, or self-priming, when connected to a reservoir comprising a fluid to be pumped, it is still left open how the pump should be operated to be primed in an efficient and controlled way when connected to a reservoir.
Having regard to the above discussion of known micropumps, it is an object of the present invention to provide a pump and components therefore which overcome one or more of the identified deficiencies and which can be manufactured in a cost-effective manner.